Saudi Journal of Kidney Diseases and Transplantation

ORIGINAL ARTICLE
Year
: 2009  |  Volume : 20  |  Issue : 3  |  Page : 443--447

Comparison of 24-hour urinary protein and protein-to-creatinine ratio in the assessment of proteinuria


Ayman M Wahbeh, Mohammad H Ewais, Mahamed E Elsharif 
 Division of Nephrology, Department of Internal Medicine, University of Jordan, Amman, Jordan

Correspondence Address:
Ayman M Wahbeh
Assistant Professor of Medicine, University of Jordan, P.O. Box 1374, Amman 11941
Jordan

Abstract

To determine the correlation between protein-to-creatinine ratio (PCR) and 24-hour urinary protein (UP), we measured proteinuria in 68 patients attending the nephrology clinic at Jordan University Hospital by 24-hour urine protein excretion and protein-to-creatinine ratio. The cutoff values for spot urine protein-to-creatinine ratio in predicting 24-hour protein «DQ»threshold«DQ» excretion of 0.5, 1.0 and 3.5 g/day were determined using receiver operating characteristic curves. A very good correlation (r= 0.832, P< 0.0001) was found between spot urine protein-to-creatinine ratio and 24-hour urine protein excretion. Bland-Altman plot showed the two tests had reasonable limits of agreement at low level of protein excretion but the limits became wider as the protein excretion increased. For protein excretion < 2.0 g/day, the limits of agreement of spot urine (PCR) and (UP) were +1.48 and -1.2 g/day. The spot urine protein-to-creatinine ratios of 0.72 (sensitivity 0.97; specificity 1.0), 1.2 (0.97; 0.89) and 3.23 (1.0; 0.86) mg/mg reliably predicted 24-hour urine total protein equivalent «DQ»thresholds«DQ» of 0.5, 1.0 and 3.5 g/day, respectively. We conclude that the protein-to-creatinine ratio in spot urine specimens is an accurate, convenient, and reliable method to estimate the protein excretion in urine. However, the protein-to-creatinine ratio will likely be within clinically acceptable limits only when proteinuria is at reasonably low levels.



How to cite this article:
Wahbeh AM, Ewais MH, Elsharif ME. Comparison of 24-hour urinary protein and protein-to-creatinine ratio in the assessment of proteinuria.Saudi J Kidney Dis Transpl 2009;20:443-447


How to cite this URL:
Wahbeh AM, Ewais MH, Elsharif ME. Comparison of 24-hour urinary protein and protein-to-creatinine ratio in the assessment of proteinuria. Saudi J Kidney Dis Transpl [serial online] 2009 [cited 2020 May 26 ];20:443-447
Available from: http://www.sjkdt.org/text.asp?2009/20/3/443/50776


Full Text

 Introduction



Measurement of protein excretion in a 24-hour urinary collection (UP) is the gold standard for the quantitative evaluation of proteinuria. How­ever, this method is cumbersome and inconve­nient, since it is difficult to collect a complete 24­hour urine sample accurately, especially in the out­patient setting. An alternative method for quan­titative evaluation of proteinuria is the measure­ment of protein-to-creatinine ratio (PCR) in an untimed spot urine specimen, which provides a more convenient method to assess protein ex­cretion and is recommended by NKF K/DOQI guidelines. [1]

Although there is moderate to high correlation between PCR and UP, [2],[3],[4],[5],[6],[7] the agreement between these two measuring techniques should be as­sessed when considering replacing one another. [8] Few studies have tested this agreement with va­riable results. [9],[10],[11]

We aim in the present study to correlate the PCR and UP agreement, and attempt to find a discriminant value for PCR that reliably deter­mines a significant threshold level of proteinuria.

 Subjects and Methods



Sixty-eight outpatients with proteinuria atten­ding the nephrology outpatient clinic at Jordan University Hospital were evaluated. Although all patients were given clear instructions how to collect the urine accurately, 18 patients were ex­cluded because the urine collection was ina­dequate in 16 and over collected in two. The re­maining 50 patients (29 men and 21 women) were included. Their average age was 51.1 ±17.0 years. The mean protein excretion at the time of the study was 2.35 ±2.47 gm and the mean PCR was 2.64 ±2.75 mg/mg. The specimens of 24­hour urine collections and random urine speci­mens were collected within two days period.

The concentration of total protein in urine was measured by the turbidometric assay using benze­thonium chloride, and the urine creatinine was measured by a creatinine Jaffe test using Ro­che/Hitachi 917 analyzer.

Statistical Analysis

Data analysis was performed using MedCalc statistical software version 9.4.2.0 (demo ver­sion). Spearman's correlation between the spot urine protein-to-creatinine ratio (PCR) and 24­hour urine total protein (UP) was used. The limits of agreement between the two methods were analyzed by the Bland-Altman method, [8],[12] and inter-rater agreement Kappa. The discriminant cutoff values, sensitivity, and specificity of PCR were tested for predicting 24-hour protein excre­tion "threshold" of 0.5, 1.0 and 3.5 g/day by re­ceiver operating characteristic (ROC) curves. [13]

 Results



There was a very good correlation between spot urine (PCR) and 24-hour urine total protein (UP) (r= 0.832, P= 0.0001), [Figure 1]. Wide deviation from the line of identity was noticed at high pro­tein excretion levels. As shown on Bland-Altman plot, the limits of agreement between PCR and UP were wide at high levels of protein excretion. These limits were better and similar across a wide range of protein excretion when data were log-transformed, [Figure 2]. For protein excretion [11],[14],[15] renal transplant, [2],[3],[16] and pregnancy. [17] The NKF K/DOQI guidelines suggests that untimed spot urine samples should be used to detect and monitor proteinuria in chil­dren and adults, it prefers a first-morning sample, but accepts a random sample if a first-morning specimen is not available.

The findings of our study showed a very good correlation between UP and PCR. To use both tests interchangeably, it is important to demons­trate that both methods agree sufficiently. Few studies have previously assessed agreements ra­ther than correlations between these tests and found wide limits. [9],[10],[11] In our study, the limits of agreement were also wide, but similar across a wide range of protein excretion when data were log-transformed, the absolute difference between PCR and UP becomes very large as protein ex­cretion increases. The inter-rater agreement Ka­ppa was calculated and was 0.585 (KW= 0 of 0.087 and KW # 0 of 0.053) indicating moderate agreement between both methods.

Urinary protein excretion is not constant and daily excretion varies by as much as 40% besides repeated 24-hour urine protein excretion varies by at least 15%. [9] Rodby et al. repeated measure­ments on 33 patients at least three months apart and found discordant results were the PCR in­creased in some patients whereas the UP fell, and vice versa. [18] Agrawal found a day to day varia­bility in 24-hour urinary protein excretion of 10% and in protein-to-creatinine ratio of 2%. [19] This variability is a likely reason for the poor agree­ment between the two methods of assessing proteinuria.

By using the ROC curves, the PCR of 0.72, 1.2 and 3.28 mg/mg in spot urine specimens represents the best threshold to reliably detect urine protein excretion of 0.5, 1 and 3.5 g in 24-hour collec­tions respectively, with high sensitivity, specifi­city, and area under ROC curve. The sensitivity and specificity in the present study are consistent with the previously published reports. [20],[21],[22],[23],[24]

We conclude that the PCR in spot urine spe­cimens is an accurate, convenient, and reliable method to estimate the protein excretion in urine. However, when the exact amount of protein ex­cretion is required, then a PCR will likely be within clinically acceptable limits only when pro­teinuria is at reasonably low levels.

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